<- old reply account i never used
It can be 1 of 2 thing, maybe scripted in world rez appearance, unlikely.
Or maybe it is saved joint positions.
If it is joint position I do not support it's use for something like clothing, it can clash with other outfit parts using joint positions. Wear enough and your animations will twitch every few second.
Go T pose as much as you can!
If it is not either then I do not know